The African Replacement Model and Multi-Regional Model of Human Evolution

By: Brad J.B. - Truman State U.

The African replacement model and multi-regional model of human evolution are two main hypotheses regarding the origin of Homo sapiens. The African replacement model (or out of Africa, OOA model) suggests that Homo sapiens evolved in Africa between 100,000-200,000 years ago and then migrated to Europe and Asia. In doing so, they gradually eliminated the other hominid species, such as the Neanderthals in Europe, without hybridization. In contrast, the multi-regional model suggests that the species Homo erectus and Homo ergaster left African approximately one million years ago. Humans then evolved from one of these species, separately on the different continents. There was, however, gene flow between Homo sapien populations during their evolution, thereby preserving the continuity of the species.

There exists evidence both for and against these two models. In 1993, David Frayer et al. provided archeological and paleontological evidence that supports the multi-regional model. The researchers contended that if Homo sapiens were capable of replacing one or more tool-using hunter-gather species already well established in Europe and Asia, then they must have had significantly superior tools and weapons. However, the archeological record does not appear to show that Homo sapiens had any technological advantage, and there is no evidence of abrupt technological changes in Europe, as may be predicated by the African replacement model.

Frayer and his colleagues also argued that, under the African replacement model, the fossil record should indicate that the archaic Homo species in Europe and Asia underwent distinctive physical alterations upon the arrival of Homo sapiens. These alterations, however, do not seem to have occurred, as the distinctive characteristics of local archaic Homo species appear to have remained constant for hundreds of thousands of years. For example, the one-million-year-old Homo erectus fossils found in Java have a prominent browridge — a distinctive characteristic of their modern contemporaries, such as the Australian aborigines (it is believed that their ancestors left Java for Australia approximately 60,000 years ago). In addition, many contemporary Chinese have distinctive shovel-shaped upper front teeth, a trait common to nearly every fossil specimen of Asian Homo erectus and Homo sapiens.

Diana Waddle (1994) and Daniel Lieberman (1995) tested the predictions of the two models using a statistical and cladistic approach. Using 12 characters to construct their cladogram, Waddle and Lieberman determined that modern humans are more closely related to archaic forms from Africa than the regional groups are to local archaic forms. Thus, in support of the African replacement model, modern humans appear to be more closely related to one another than they are to any local archaic species. If so, then the existence of enduring traits described by Frayer and colleagues must have been the result of convergent evolution in H. erectus and H. sapiens.

In addition to archeological and paleontological evidence, genetic data is currently being used to test the accuracy of each model. By sequencing particular genes extracted from both modern and archaic humans, a phylogeny has been estimated to see if it most closely resembles the cladogram predicted by the African replacement model or the multi-regional model. Researchers recently collected mitochondrial DNA from the skeleton of a Homo neanderthalensis that lived in Germany approximately 30,000 – 100, 000 years ago. A specific sequence from the neanderthalensis was then compared to the same sequence in 663 modern humans from all continents, 7 chimpanzees, and 2 bonobos. Using this data to construct a phylogeny, it was found that all modern humans, located on every continent, were more closely related to each other than to the archaic European neanderthalensis. This clearly suggests that Homo sapiens evolved in Africa before migrating to the various continents, as indicated by the African replacement model.

In 1992, S. Blair Hodges and colleagues analyzed sequences of non-coding mitochondrial DNA from 189 living humans from a variety of regions. Since mitochondrial DNA is inherited maternally, Hedges mitochondrial evolutionary tree depicts the direct maternal ancestry (the mitochondrial DNA inherited from a person's mother's father is considered indirect maternal ancestry) of the 189 individuals all the way back to a single woman. The earliest branches of the tree clearly show an abundance of African lineages, indicating that the common maternal ancestor lived in Africa. Furthermore, it has been estimated from the sequences of entire mitochondrial genomes that the common ancestor of all modern mitochondrial DNAs lived between 125,000 – 161,000 years ago. However, other estimates have been developed using approximately the same techniques, and suggest that the common ancestor could have lived anywhere between 129,000 and 536,000 years ago. Despite the relatively wide temporal range, this data most strongly supports the African replacement model.

Finally, an additional study by Bowcock et al. entailed the comparison of 30 nuclear microsatellite loci from individuals in 14 different populations. These loci are unique in that they contain short strings of nucleotides (about 2-5 bp in length) that are repeated in tandem. The exact number of repeats within a single locus is highly distinct among different individuals, and thus there are many different alleles for each locus. Bowcock and colleagues determined the “multilocus genetic distances” between the populations based on the allele frequencies at each of the 30 microsatellite loci. The genetic distances were then used to estimate a phylogenetic tree of the 14 populations. The first branch on the tree separates African from non-African populations, while populations that are closest geographically are also nearest to one another on the tree. Using this same data, D.B. Goldstein and Colleagues determined that the African and non-African populations diverged approximately 75,000 – 287,000 years ago. This data supports the African replacement theory and the previous genetic studies with mitochondrial DNA. It seems unlikely that there would have been enough gene flow under the multi-regional model to account for the maintenance of region-specific characters over a time period of one million years or greater.

Overall, I believe that the African replacement theory, thus far, has the larger and most convincing body of evidence. In fact, the only evidence presented in support of the multi-regional model (referring to the archeological and paleontological evidence suggested by David Frayer) does not seem to be highly persuasive. For example, the fact that Homo sapiens do not appear to have had a substantial technological advantage over the hominids they encountered upon expansion does not prove that they did not replace them. There are many other factors that may have played a role in the survival of Homo sapiens, such as their unique intellectual capabilities. There are simply too many uncertainties involved in this argument to be convincing. In regard to the continuity of distinctive regional traits, such the strong browridge found among H. erectus fossils from Java and present-day Australian aborigines, the theory seems reasonable, yet far from conclusive. In contrast, the ever-increasing molecular evidence strongly favors the African replacement model, suggesting that Homo sapiens left African approximately 100,000 – 200,000 years ago. This data appears to be both reliable and accurate as the analysis of different genes sequences have lead to approximately the same conclusion regarding the origin of Homo sapiens.